1. Field of the Invention
The present invention relates to a control circuit applicable to a power converter and its operation method, and more particularly, to a control circuit and its operation method capable of reducing power consumption of a power converter.
2. Description of the Prior Art
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a diagram showing a power converter and a control circuit of the prior art. FIG. 2 is a diagram showing waveforms of related signals of the power converter and the control circuit in FIG. 1. As shown in FIG. 1, the power converter 10 comprises a primary winding PR, an auxiliary winding AUX, a power switch M, a diode D and a capacitor C1. The power switch M is coupled between the primary winding PR and ground. The diode D and the capacitor C1 are coupled to an output end OUT. The control circuit 100 is coupled to the auxiliary winding AUX and a gate end of the power switch M. The control circuit 100 is configured to turn on the power switch M according to an auxiliary voltage of the auxiliary winding AUX, and turn off the power switch M according to a duty cycle, in order to generate an output voltage Vout at the output end OUT of the power converter 10. As shown in FIG. 2, when the power converter 10 is operated in a discontinuous conduction mode (DCM), in order to reduce power consumption of the power converter 10, the control circuit 100 must turn on the power switch M at a time point Ton when the auxiliary voltage Vaux reaches a wave trough after beginning resonance (such as at time point T1), thus the power converter 10 can have smaller power consumption.
The control circuit 100 of the prior art determines the time point Ton when the auxiliary voltage Vaux reaches the wave trough after beginning resonance according to ideal values of circuit components of the power converter 10. However, parameters of the circuit components may have tolerances, such that the resonance frequency of the auxiliary voltage Vaux is affected by the tolerances, that is to say, the time point when the auxiliary voltage reaches the wave trough after beginning resonance may be different in different power converter. The control circuit 100 of the prior art cannot precisely determine the time point Ton when the auxiliary voltage Vaux reaches the wave trough after beginning resonance, such that the power converter 10 of the prior art has larger power consumption.